1. Field of the Invention
The present invention concerns passively controlled liquid crystal displays wherein the orientation layer arranged on the substrate plates impresses a large pretilt angle, as well as a method of producing such liquid crystal displays.
2. Description of the Prior Art
Liquid crystal displays have been known for some time in the state of the art. They have the common feature of consisting of two transparent plates that run parallel to each other and at a given distance between them, as well as of a frame that rigidly joins these plates, the space delimited by the frame and the two plates being filled with liquid crystal. Especially in high-quality liquid crystal displays suitable for use in television sets, the two plates will be made of glass. The plate surfaces facing each other are coated with a variety of different materials.
Thus, in the case of monochromatic liquid crystal displays, where the two plate surfaces facing each other are provided with the same sequence of coatings, it is known first to provide the said plate surfaces with a transparent and structured electrode array, possibly as an ITO arrangement. This electrode layer is followed by an equalization and insulation layer, which may be a layer of silicon dioxide for example. The equalization and insulation layer carries the socalled orientation layer. This orientation layer, which is in direct contact with the liquid crystal, is traditionally made of polyimides or polyphenyls. With a view to improving the orientation of the liquid crystal molecules immediately adjacent to the orientation layer, the surface of the orientation layer facing the liquid crystal can also be oriented by means of mechanical processes.
When the liquid crystal display is to be capable of reproducing colors, one of the two substrate plates, preferably the substrate plate on which the reproduction of--say--the television image is subsequently to be obtained, will be provided with an additional layer between the surface of the glass plate facing the liquid crystal and the electrode layer. This layer, which contains the organic dyestuffs, will henceforth be referred to as the chromophore layer.
High-resolution, passively controlled liquid crystal displays, which have a wide angle of view and have to be highly multiplexed, call for orientation layers that impress a large angle of incidence, the socalled pretilt angle, upon the liquid crystal molecules.
Orientation layers having the required properties can be produced by means of oblique vacuum deposition of silicon dioxide on the substrate plate. Very large pretilt angles can be set when this technique is employed. A disadvantageous feature is however constituted by the fact that this method is very costly and, what is more, can be used only with very small substrate plates. When larger substrate plates have to be produced with a pretilt angle up to about 25.degree., it is known that organic orientation layers, possibly a polyphenyl layer, can be applied to the formation of a polyphenyl layer on a substrate plate calls for tempering conditions of the order of 400.degree. C. and all known dyestuffs from which a given chromophore layer could be formed will change their previously stable color above a temperature of about 240.degree. C. to 250.degree. C., intensive efforts were subsequently made--bearing in mind that the chromophore layer is arranged between the surface of the glass plate and the electrode layer and that it must therefore quite inevitably be heated to about 400.degree. C. when the orientation layer is formed--to lower the tempering temperatures to a value that would not damage the chromophore layers. This requirement was satisfied by the development of the socalled low-temperature polyimides and low-temperature polyphenyls, which only call for a tempering temperature of the order of 160.degree. C. to 260.degree. C. Although the liquid crystal displays produced by employing these substances are already characterized by good color reproduction properties, further investigations have shown that orientation layers formed with low-temperature polyimides and/or low-temperature polyphenyls have pretilt angles of up to 8.degree. at the very most. Attempts to combine a substrate plate devoid of a chromophore layer but provided with a high-tilt orientation layer tempered at a temperature of about 400.degree. C. with a substrate plate having an orientation layer made up of low-temperature polyimides have shown that the total tilt that can be obtained between the two substrate plates rises above the individual tilt in the orientation layer constituted by the low-temperature polyimides. But even the total tilt of these combined substrate plates, which is not equal to the arithmetic mean of the individual tilts but merely approximates the individual tilt of the orientation layer having the larger coupling energy, is not yet sufficient for high-resolution liquid crystal displays. The present invention involves a liquid crystal display, and more particularly an orientation layer, that can be easily produced and will impress a large pretilt angle upon the liquid crystal molecules, as well as a method of producing an orientation layer that will guarantee a large pretilt angle.